Conventionally, crustal movement observation by gauging ground strain or the like has been performed for the purpose of prediction of earthquakes or the like. As a method of gauging ground strain, for example, as a strain sensor disclosed in Patent Document 1, a method in which a sensor whose inside is filled with fluid is inserted into a hole provided on land, to sense a change in the volume in the hole due to a change in the shape of the hole by the inside fluid has been known. Moreover, in recent years, it is considered that a change in stress in the crust is understood in more detail by, not only gauging strain on land, but also gauging strain at the bottom of water such as the ocean floor.